There exist many communication systems designed for instant or near-instant communication between users, for example, radio, cellular telephones, satellite telephones, IP-telephony, instant messenger, video conferencing, broadband telephones, cable telephones and land-line telephones. This list is not exhaustive. In each of these communication methods, there are often multiple standards.
In order for one person to successfully communicate with another person using a communication system, both persons need to have access to an interface of the communication system. In addition, the initiator of the communication needs to have a handle or address of the second person. Typically, there exist directories listing the handles (e.g., telephone number) of the other person. Currently, a person may have multiple handles, including, cellular telephone number, land line number, instant messenger user ID, IP-telephony type, user ID and e-mail address.
If a person does not have a handle for the communication system which he wishes to use for communication, the person needs to register for that system, which may include various activities which take time, effort and provision of identification information.
One example of a typical registration process is found in Skype, a VOIP service. Similar processes are found in MSN and other instant messenger applications. In a typical usage, a first person sends an e-mail to a second person inviting him to sign up for Skype. This e-mail is sent via the Skype interface or as a regular e-mail mentioning the specific network ID/handle name (e.g., Skype name, ICQ #, hotmail/Gmail/Yahoo ID/e-mail address) of the e-mail sending/inviting user. The receiver of the e-mail can click on a button in the e-mail to open a download and/or registration page on a Skype portal. The registration is to a closed network A closed network requires that you have an ID/name that allows access to the specific network. The receiver downloads the Skype application/client, installs it, and runs the application to register as a Skype user. Once this process is completed, the receiver may call back the initiator of the process using his installed Skype client, after entering the initiator's ID manually. In addition, each person that the receiver wants to communicate with needs to be found in the closed network or added to a contact list and be invited—in some cases this initiates the e-mail invitation process described above.
Some systems allow a shortened registration process. For example, the Google-talk instant messenger system allows a user to initiate/invite a user from a Google-mail user account. If the invited user is part of the closed group network (Gmail/Google Talk) and has a Gmail ID then the registration process is not needed. However, in this case, as in others, if the invited user doesn't have a Gmail ID/e-mail address, the invited user needs to join/register to the Gmail network and get a new Gmail ID.
It should be noted that e-mail is a unique personal communication network on the public Internet. Following are particular features of e-mail:                1. Has no significant competing networks that use the same communication type (e-mail)        2. Is based on open standards.        3. Is based on the Internet's universal (distributed) DNS database.        4. Anyone can get an Internet domain (so there are many providers of the service).        5. Any Internet domain can enable e-mail for it's domain by adding the domain's communication (e-mail) servers to the Internet's universal (distributed) DNS database e-mail specific entries.        6. The domain owner is responsible for the domain's communication (e-mail) servers.        7. The domain owner is responsible for the personal (e-mail) addresses under that domain and therefore can add any personal (e-mail) address he wishes under that domain.        8. Is the most popular personal communication network (by number of users with personal e-mail addresses) due to the above and due to the fact that:                    a. It is a mature personal communication network that started many years ago.            b. Almost all Internet domain are open for e-mail communication, and even more so for domains belonging to organizations.            c. The existing e-mail communication systems (both client and server) are highly compatible due to the maturity of the network.            d. Anyone can get a personal e-mail address, either by:                            i. Starting one's own domain.                ii. Joining an ISP—almost any ISP provides one or more e-mail addresses to anyone who joins its service—for free.                iii. Join one of the numerous services that provide free e-mail accounts (Hotmail, Yahoo, Gmail, etc.).                                    e. Almost any person belonging to an organization with an “e-mail enabled” domain gets an e-mail address under that domain (and usually uses it in his/her day-to-day activities).                        9. Users within the network can communicate with each other, irrespective of the manner/provider in which they access the network.        
Thus e-mail is an example of an Internet communication network that basically anyone can join, and without needing to register specific handles with any central authority. The telephone (including satellite and cellular) network has similar properties.
In comparison to other personal internet communication methods:                1. All the popular Instant Messaging communication networks on the public Internet (MSN, Yahoo, Google) require registration for their closed networks.        2. The Skype VOIP and IM service similarly requires registration to its closed network.        3. Jabber doesn't match at least items 1, 8 and 9 (is not widespread enough for regular users to trust for their communication needs, especially considering the closed networks).        4. The standardized SIP protocol (both for VOIP and IM) doesn't match at least items 1, 8 and 9 (is not widespread enough for regular users to trust for their communication needs, especially considering the closed networks).        